1. Field of the Invention
The present invention relates to the forming of recesses, bores, or trenches in a material, and especially applies to the field of semiconductor materials, intended for the forming of discrete or integrated components.
The present invention more specifically relates to the forming of recesses, bores, or trenches which are oblique with respect to a main surface of the material to be etched and have a small transverse dimension.
2. Discussion of the Related Art
When recesses are desired to be etched according to a pattern having at least one very small dimension (smaller than 10 μm and possibly smaller than 1 μm) into the surface of a material, one of the currently-used methods, especially in the field of microelectronics, is a plasma etch method.
FIG. 1 very schematically shows a plasma etch reactor. The reactor comprises an enclosure 1 provided with a bottom 2 on which is placed a support or susceptor 3, that may be isolated from bottom 2. A wafer 4 in which etch operations are desired to be performed is placed on support 3. A plasma 5 is created by any means in the enclosure, for example, by a radio frequency field, and may be confined by electromagnetic fields. Of course, the enclosure further comprises means, not shown, of gas introduction, circulation, and extraction, and possibly means for imposing a temperature, for example, for cooling down, to support 3, as well as means for biasing this support. As schematically illustrated in FIG. 1, the plasma generally extends into the entire enclosure, and by all means in the vicinity of wafer 4 to be etched, and is separated from all the internal enclosure surfaces by a distance s, in which a neutral area, called a sheath, is present. In the case where wafer 4 is a silicon wafer, plasmas are often obtained from fluorinated or chlorinated gases.
Generally, anisotropic plasma etch methods involve two mechanisms: an etch mechanism and a passivation mechanism that may be alternated or not. The etch mechanism is considered as involving two phenomena, on the one hand a bombarding by the ions present in the plasma, on the other hand a chemical etch by reactive substances present in the plasma. The two phenomena are indissociable. The ion bombarding is used to eliminate the reaction products arranged at the bottom of a recess being formed.
These explanations have been given to expose the context of the present invention, given that plasma etch methods are well known per se and that there exist many variations thereof that can be found in specialized works.
FIG. 2 is a cross-section view of a wafer 4, coated with a masking layer 7 comprising openings 8, arranged in the plasma etch reactor of FIG. 1, that is, it shows a plasma region 5 separated from the wafer by a sheath thickness s.
The ions bombard the silicon wafer perpendicularly to the surface thereof and are active, with chemical species located in the plasma, at the level of openings 8 in mask 7 to etch in wafer 4 recesses 10 orthogonal to the wafer surface.
Thus, anisotropic plasma etch techniques do normally not enable forming recesses which are not orthogonal to a main surface of a wafer to be processed.
Japanese patent application 2002-134470 suggests to deviate ions due to a lateral field created at the surface of a wafer. This field results from a charge accumulation under insulating layers having different thicknesses, the thickness difference being of about 100 nm. This method is not adapted to provide deep trenches nor substantial inclination angles.
So, if oblique recesses or trenches are desired to be formed, techniques other than plasma etching are used. An oblique trench will for example be formed by sawing with diamond saws (JP-A-56054066), by sand blasting (JP-A-03225961), or with a laser (EP-A-0070692), or also by slanted ionic etching (JP-A-08130220 and JP-A-62173737). The methods do not provide thin trenches or enable forming patterns of complex shapes.